1. Field of the Invention
The present invention concerns a method of clamping blades of turbomachines in order to influence the blade vibrations occurring in such machines, the blades, being each provided with a shroud plate.
2. Dissussion of the Background
In turbomachines, the rotating blades are excited by, among other things, irregular incident flow. This excitation force often causes unallowable alternating stresses in the blades. In order to combat these dangerous vibrations, thickening the blade section can be considered as an obvious measure. This measure, however, causes a substantial deterioration in the efficiency and, for this reason, is not used.
Wherever it is possible to provide the blades with shroud plates, these are themselves used for damping an/or frequency modification and for stiffening. For this purpose, the flanks of the shroud plates are machined in such a way that they form surfaces of various shapes in contact with one another.
Although these measures exhibit certain advantages relative to the use of damping wires and pins, these shroud plate designs are subject to various other disadvantages, such as:
expensive machining of the contact surfaces
complicated assembly
varying contact surface forces depending on the operating condition
mechanical wear of the surfaces in contact with one
another, so that the desired damping continually deteriorates.
In order to help deal with these disadvantages, it has become the practice to install the blades with pre-torsion. For this purpose, the rhomboid-shaped shroud plates are provided with an excess dimension relative to the theoretically freely available pitch dimension so that the individual shroud plates and blade sections experience elastic deformation in the course of assembly. This is done by the application of a peripheral force and the blades are thus made to adopt a compact connection.
The damping and stiffening of the row of blades achievable by this means is, in itself, of good quality; with such a technique, however, it is found that both the assembly and removal of the blades is associated with disadvantages, particularly with respect to the assembly of the last blade of a group, where a large amount of effort has to be employed. In order to apply the peripheral force necessary for this purpose, special force generating equipment with a large force capability must be provided because of the relatively narrow space conditions there present. At times, however, this equipment is only suitable for certain types of machine so that the expense in each case is not unsubstantial.